Web crawlers can trigger the creation of many thousands of sessions as they
crawl a site which may result in significant memory consumption. This Valve
ensures that crawlers are associated with a single session - just like normal
users - regardless of whether or not they provide a session token with their
requests.

Specify the regular expression (using java.util.regex.Pattern) that will be used
to identify crawlers based in the User-Agent header provided. The default
is ".*GoogleBot.*|.*bingbot.*|.*Yahoo! Slurp.*"